Lithium secondary batteries, which are light in weight and have a high energy density, are widely used mainly as a power source for portable devices. A typical lithium secondary battery comprises an electrode group, a non-aqueous electrolyte and a battery can for housing the electrode group and the non-aqueous electrolyte. The electrode group includes a positive electrode, a negative electrode and a separator. The positive electrode comprises a strip-shaped positive electrode current collector and a positive electrode material mixture layer carried on the positive electrode current collector. The negative electrode comprises a strip-shaped negative electrode current collector and a negative electrode material mixture layer carried on the negative electrode current collector. The positive electrode and the negative electrode are spirally wound with the separator interposed therebetween. The separator functions to insulate the positive electrode and the negative electrode from each other and to retain the non-aqueous electrolyte. The separator is usually a microporous film. The microporous film is produced by forming a polyolefin (e.g., polyethylene or polypropylene) resin into a sheet.
A typical microporous film tends to shrink upon heating because it comprises a polyolefin. Accordingly, if a lithium secondary battery is left in an extremely high temperature environment for a long period of time, the separator contained in the battery will deform. As a result, the positive electrode and the negative electrode physically come in contact with each other, causing an internal short-circuit. Once an internal short-circuit occurs, Joule heat is caused by the short-circuit current, which facilitates the deformation of the separator and allows the shorted area to enlarge. This can cause the battery to overheat.
In order to attain higher capacity lithium secondary batteries, thinner separators are being developed. However, the occurrence of internal short-circuit increases as the thickness of separators is decreased. Accordingly, prevention of an shorted area from enlarging in the event of an internal short-circuit is an important issue. Under the circumstances, Japanese Patent No. 3371301 (i.e., Japanese Laid-Open Patent Publication No. Hei 7-220759) proposes to form a porous insulating layer comprising an inorganic filler (solid fine particles) and a binder on an electrode surface. The porous insulating layer is filled with an inorganic filler such as alumina or silica. The filler particles are bonded by a relatively small amount of binder. Such porous insulating layer is difficult to shrink even at high temperatures. This prevents a shorted area from enlarging even in the event of an internal short-circuit, avoiding the overheating of the battery.
When a porous insulating layer is formed on an electrode surface as disclosed by Japanese Patent No. 3371301, because the porous insulating layer is rigid, the produced electrode will be less flexible. Moreover, a portion of the electrode group having a large curvature (e.g., an inner portion of an electrode group or a bend of an electrode group for a prismatic battery) will have a relatively low strength. If such electrode group is pierced with a nail through the low strength portion, the electrodes and separator will break easily and the shorted area will enlarge easily. Consequently, the overheating of battery may not be prevented.